<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      .up::after,
      .down::after {
        display: block;
        content: "";
        clear: both;
      }
      .up > div {
        float: left;
        width: 0;
        border: 50px solid transparent;
      }
      .down > div {
        float: left;
        width: 0;
        border: 50px solid transparent;
      }
      .up > div:nth-child(1) {
        border-bottom-color: yellow;
      }
      .up > div:nth-child(2) {
        border-left-color: yellow;
      }
      .down > div:nth-child(1) {
        border-right-color: yellow;
      }
      .down > div:nth-child(2) {
        border-top-color: yellow;
      }
    </style>
  </head>
  <body>
    <div class="up">
      <div></div>
      <div></div>
    </div>
    <div class="down">
      <div></div>
      <div></div>
    </div>
  </body>
</html>
